The only way this will work is if you use light/infrared. That is the only way you can get a "radio beam" where the angle subtended by the beam is narrow enough to select one person standing next (3' apart?) to another at a distance of ~40'. Do the Trig.
I think there's lots of different ways to transport the radiative illumination to the targets:
Best I can think of is
CW Tone-Modulated IR laser + diverger optics + parabolic reflector. The IR energy concentration uW / cm2 made low enough not to be a risk to eyesight (even looking straight into the dish), yet high enough to activate a tuned super-regenerative reciever in each RX. The attenuation of the broad area beam over distance is minimal if collimated to the same size of the dish.
If you convert a standard car reflector lamp may be a cheaper possible. Perhaps use a defocused laser, or wide angle NIR LED.
Maybe a modified battery torch can be adapted to provide most of optics.
Or cheap dirty way could be an xy array of narrow-angle NIR LED's, driven by CW tone-modulation.
Reciever: The battery-powered Detector can contain a photodiode, a tuned circuit and 2-transistor regenerative amplifier feeding the LED directly (operating in class-c mode when energised
)
Probably a fet, a bjt, 4 resistors, 3 caps, inductor, and a photodiode. Oh and an LED and battery
Anyone wanting to patent this, please remember: you saw this here first!